This invention relates to a high-temperature, low-density structural insulating composite having a favorable combination of low thermal conductivity, high compressive strength and elevated service temperature. Composites of the invention have properties useful as insulating structures at high temperatures and particularly as liners within cylinders of high temperature engines.
In the development of high temperature liners, available materials have in general not exhibited the desired combination of thermal conductivity and compressive strengths at operating temperatures of 800.degree. C. and above. The thermal conductivity of amorphous glass is in the order of 1.2 Watts/meter/.degree.C.(W/m.degree.C.). A commercial glass ceramic with voids has a thermal conductivity in the order of 0.6 W/m.degree.C. At this value (0.6 W/m.degree.C.) of conductivity, calculations indicate that the radial heat conduction through the ceramic is roughly equal to the axial heat conduction through the metal wall which the ceramic is intended to replace for thermal purposes. Calculations further indicate that efficiency of a heat engine and particularly a Stirling engine would be improved if the thermal conductivity of the insulation could be lowered to about 0.2 W/m.degree.C.
While some materials have the lower thermal conductivity, they usually have a useful service temperature below 800.degree. C. and/or insufficient compressive strength. In many instances, low thermal conductivity is combined with low strength or high thermal conductivity is combined with high strength. A material having a combination of a thermal conductivity below about 0.20 W/m.degree.C. and a strength above about 250 psi and particularly above about 400 psi would be desirable.
Accordingly, one object of the invention is a high temperature insulating material. Another object is a high temperature insulating material capable of being formed into a structural member. An additional object is a high temperature insulating material having a thermal conductivity below about 0.2 W/m.degree.C., a compressive strength above about 250 psi and a useful service temperature of above about 800.degree. C. These and other objects will become apparent from the following description.